The mean physical and optical properties of regional haze dominated by biomass burning aerosol measured from the C‐130 aircraft during SAFARI 2000

Abstract

[1] Instrumentation on the Met Office C-130 aircraft measured aerosol physical and optical properties during the Southern African Regional Science Initiative (SAFARI 2000) in September 2002 while flying from Windhoek, Namibia. Filter measurements of aged regional haze suggest a ratio of apparent elemental carbon (EC a ) to organic carbon (OC) of 0.12 ± 0.02 and mass fractions of 5% EC a , 25% inorganic compounds, and 70% organic matter (OC plus associated elements). The submicron size distribution of aged regional haze may be fitted with three lognormal distributions with geometric mean radii (r n ) of 0.12 ± 0.01, 0.26 ± 0.01, and 0.80 ± 0.01 mm and geometric standard deviations (s) of 1.3 ± 0.1, 1.5 ± 0.1, and 1.9 ± 0.4. Measurements over 2500 km from the emission region show similar r n and s for the smallest two modes, while the third mode is absent presumably as a result of sedimentation. At a wavelength (l) of 0.55 mm, effective medium approximations suggest a refractive index of 1.54 À 0.018i for aged regional haze aerosol. The single scattering albedo (w ol ) derived using this refractive index and measured size distributions are consistent with those from the nephelometer and Particle Soot Absorption Photometer (PSAP). The optical parameters for aged regional haze a few days old are specific extinction coefficient (k el=0.55 ) of 5.0 ± 0.8 m 2 g À1, asymmetry factor (g l=0.55 ) of 0.59 ± 0.02, and w ol=0.55 of 0.91 ± 0.04. Measurements of fresh biomass burning aerosol a few minutes old show smaller more absorbing particles. Vertical profiles of carbon monoxide, aerosol concentration, and aerosol scattering show a good correlation. Over land, aerosols become well mixed in the vertical from the surface to approximately 500 hPa. Over ocean, the aerosols can be separated from underlying stratocumulus cloud by a clear gap and a strong inversion, which may limit the indirect effect.

The effect of smoke, dust, and pollution aerosol on shallow cloud development over the Atlantic Ocean

“…The smoke and pollution effect on cloud cover generates a radiative forcing at the top of atmosphere that is about half of today's aerosol effect or Ϫ6 W͞m 2 , and reduction of sunlight available for evaporation from the ocean by 7 W͞m 2 , thus dominating climate change in this region. This finding is in line with the observation of a global dimming of sunlight at the surface over the land in the last 50 years (48) of 10-20 W͞m 2 , taking into account the higher aerosol concentrations over the land, near the sources, than over the ocean (1,49). Recent papers show that the dimming effect reversed in the mid-1980s and a brightening resumed (39,50).…”

“…Further south, Haywood et al (1) observed that the stratiform clouds are detached from the overlaying smoke layer, with a vertical separation of a few hundred meters. A recent modeling study (18) showed that under such conditions the absorption of sunlight by the smoke alone can influence the underlying stratiform clouds even without physical interaction.…”

“…Haywood et al (1,2) evaluated the cloud retrievals in the presence of African dust and smoke aerosol. They found that the MODIS droplet effective radius (using the 0.86-and the 2.1-m channels) is not affected by overlying aerosol.…”

“…The Southern Tropical Atlantic (30°S-20°S) is dominated by clean maritime air. The region between 20°S and 5°N is a relatively well defined region covered by smoke from biomass burning in Africa (1,2). The Northern Tropical Atlantic (5°N-30°N) is under heavy influx of dust from Africa (3), and the Northern Atlantic (30°N-60°N) is impacted by anthropogenic pollution aerosol from North America and Europe.…”

Measurements of aerosol properties from aircraft, satellite and ground‐based remote sensing: a case‐study from the Dust and Biomass‐burning Experiment (DABEX)

“…This compares with values of 0.85-0.90 measured during the Southern African Regional Science Initiative (SAFARI-2000) (Eck et al, 2003;Haywood et al, 2003a;Leahy et al, 2007). In contrast, the dust aerosol was very weakly absorbing with single-scattering albedos of around 0.98 at 550 nm .…”